The invention relates to an arrangement for detecting the intrusion of and identifying land vehicles when these land vehicles pass a reference plane, comprising an optical system for focussing a narrow detection beam for detecting a passive infrared radiation in said reference plane, a filter allowing the selection of the spectral analysis band and at least one detector arranged in the focal plane of said optical system.
This arrangement is suitable for civil or military uses. It is more specifically designed to identify the general shapes of mobile vehicles, in particular the proportion between some of their characteristic elements and not their speed or the direction in which they move. It is, for example, used to recognize the type of vehicles which enter or leave a parking-lot or vehicles which usually do not drive on given roads or within the precincts of a factory etc. The use for military purposes consists in the arrangement being used, in association with further pick-up devices or sensors, in the processing of an automatic light switch-on command for an anti-tank trap. The principle of detecting objects, persons or vehicles by means of passive infrared radiation detectors, more specifically piezo-electric detectors, is known and is used, for example, to trigger automatically the opening of doors when a passive infrared radiation (IR-P) is transmitted by the object in a narrow detection beam. The European Pat. 0 065 159 in particular discloses a movement detector for monitoring a space which utilizes the infrared thermal radiation of a non-authorized person entering the space and in which the radiation receiver is a piezo electric element. Passive infrared detectors (IR-P) are increasingly preferred to Doppler radar arrangements, which are much more susceptible to false alarms and which can be detected by the electromagnetic radiation they transmit, the latter constituting a drawback for military uses. The pyro-electric detectors are simple to use because of the facts that they do not require any cooling, and are suitable for a detection in a range of some dozens of metres at a maximum, beyond which the thermal noise of the detectors becomes predominant with respect to the useful signal searched for. The foregoing relates to a simple detection which does not furnish any other information than the presence or absence of a warm object in a detection beam. On the other hand, infrared thermographical systems are known which by means of a thermal camera and a television monitor render it possible to obtain an image from the passive infrared radiation in a field of vision which may be at a distance of several kilometers and in accordance with several dozens of contrast levels. Such systems which however always require cooling of the detectors, are complicated and expensive.